This invention relates to blade lattice structures for axial fluid machines, and more particularly to a blade lattice structure for an axial fluid machine, such as a steam turbine, gas turbine, axial compressor, etc., which has a defined annular flow path.
Generally, when an operating fluid flows through a blade lattice structure which constitutes a stage of an axial flow machine, the operating fluid loses its energy due to friction with the surfaces of blades and side walls constituting the blade lattice structure. These losses are generally referred to as profile losses and side wall losses, of which the side wall losses are attributed to mutual interference of boundary layers on the surfaces of the blades, boundary layers on the surfaces of the side walls and secondary flows in the flow path of the blade lattice structure, and cause a reduction to occur in stage efficiency in the axial fluid machine. It is essential to reduce the side wall losses in order to increase the efficiency of a compact axial fluid machine, particularly high and medium pressure stages of a steam turbine and a gas turbine which have a small aspect ratio (blade chord lengths/blade heights).
Proposals have hitherto been made for reducing the side wall losses. Some of these proposals include the use of following arrangements:
(1) Attaching shield plates to side walls. PA1 (2) Throttling side walls. PA1 (3) Controlling boundary layers.
It is to be noted that since the arrangements hitherto suggested are all intended to reduce secondary flows by merely considering the two dimentional characteristics of a blade lattice structure and no consideration is paid to the three dimensional characteristics of a well-known axial fluid machine or the flow characteristics of a fluid flowing through a blade lattice structure forming an annular flow path, no appreciable increase in stage efficiency has ever been obtained.
The mechanism of production of secondary flows in a blade lattice structure forming an annular flow path in a stage of an axial-flow turbine will be discussed. Considering a flow in an outlet flow path of a stationary blade, it is noted that, in an annular flow path defined by upper and lower walls of a diaphragm having the stationary blade secured thereto and an adjacent stationary blade, boundary layers formed along the wall surfaces of the diaphragm, boundary layers formed along the leading and trailing edges of the stationary blade and a cross flow directed from the trailing edge toward the leading edge of the stationary blade interfere with one another, with a result that secondary flows are produced near the upper wall surface and lower wall surface of the diaphragm. According to our experience, this phenomenon is such that the secondary flow along the upper wall surface of the diaphragm is on a larger scale than the secondary flow along the lower wall surface of the diaphragm. More specifically, owing to the secondary flows produced in a flow path defined between the two adjacent stationary blades, the total pressure loss of the stationary blade is distributed such that the losses occurring in the vicinity of the blade tip located radially outwardly of the turbine and in the vicinity of the blade root located radially inwardly of the turbine are much greater than the loss occurring in the central portion of the flow path. The phenomenon particularly noteworthy in a blade lattice structure forming an annular path is that the loss at the blade tip is greater than that at the blade root. In some cases, the loss at the blade tip amounts to several times as great as the loss at the blade root. This phenomenon should be avoided in order to increase stage efficiency.
This phenomenon has escaped one's attention in the past because the flow of a fluid in a blade lattice structure has been considered as a two dimensional phenomenon. More specifically, in an annular flow path which is a three dimensional flow, a balance should be established between the centrifugal forces exerted by a fluid and the pressure applied radially by such fluid. This makes the pressure high at the blade tip which is located radially outwardly of the turbine and low at the blade root which is located radially inwardly of the turbine. Thus, this radial differential pressure promotes the tendency of a secondary flow being produced in the vicinity of the blade tip. The secondary flow which shows a marked development at the tip of a stationary blade not only causes a marked reduction in the efficiency of the stationary blade but also worsens the condition of the flow of the fluid toward the next following movable blade. Consequently, such secondary flow would naturally reduce stage efficiency.